A Digital Television Standard published by the Advanced Television Subcommittee (ATSC) specifies vestigial sideband (VSB) signals for transmitting digital television (DTV) signals, such as high definition television (HDTV) signals. The VSB signals are transmitted in 6-MHz handwidth television channels such as those currently used in conventional terrestrial broadcasting of National Television Subcommittee (NTSC) analog television signals within the United States. The HDTV system is incompatible with the NTSC broadcast standard, thus, if the broadcasting industry immediately adopted the digital HDTV system and abandoned the NTSC system, NTSC television receivers purchased within the last few years would be rendered obsolete. To avoid this undesirable result, the transition from conventional analog NTSC-standard broadcasts to digital HDTV television broadcasts will occur over a period that may last fifteen years to allow for normal attrition of older NTSC television receivers. Thus, during this transition period, both NTSC analog and HDTV digital signals will occupy the television spectrum. Television receivers manufactured during the transition period will be capable of processing both NTSC and HDTV signals.
At particular geographic location, a receiver may receive signals from two transmitters that have adjacent channel spectrum allocations (e.g., an HDTV channel adjacent to an NTSC channel). When attempting to receive one of the signals (the desired signal), the other (the undesired, adjacent channel signal) creates interference in the system. Consequently, the signals must be filtered to reduce the interference from the adjacent channel signal. This situation is particularly problematic when the desired signal is an HDTV signal and the undesired signal is lower adjacent NTSC signal because of the proximity of the aural carrier of the NTSC signal to the digital signal. The desired to undesired (D/U) signal ratio can be more than −40 dB. This presents an extreme challenge to fabricate surface acoustic wave (SAW) filters, such as those required by digital television receivers, that have a steep transition band roll off in order to remove the undesired signal without significantly attenuating the desired signal.
Thus, there exists a need in the art for digital television receivers using conventional SAW filters that are able to reduce adjacent channel interference, particularly, lower adjacent NTSC channel interference.